@Service
public class NotificationService {
    private final SimpMessagingTemplate messaging;
    // ...
    public void broadcastAnnouncement(Notification notification) {
        messaging.convertAndSend("/topic/announcements", notification);
    }
    public void sendComment(Notification notification, String username) {
        messaging.convertAndSendToUser(username, "/queue/comments", notification);
    }
}
